In HVDC transmission systems of today, a transformer is conventionally connected between an HVDC converter and the AC grid to which the converter is connected. One purpose of such transformer is to prevent DC current interaction between the AC grid and the HVDC converter. DC currents may from time to time occur on the AC side of the converter, for example due to asymmetric switching of the converter switching elements. DC currents which are allowed to flow through the AC grid will typically cause problems in terms of saturation of transformers in the AC grid, increased risk for corrosion of network cabling, etc. For this reason, the permitted levels of DC current injected into the AC grid are very low, typically no more than a few percent of the rated AC current RMS value.
Transformers connected between an HVDC converter and an AC grid often serve further purposes, such as for example facilitating for voltage adaptation between different AC networks. However, in some circumstances, for example when the voltages of the different AC networks are well matched, voltage adaptation can be achieved by other means. By omitting the transformer from the connection between the HVDC converter and the AC grid, several advantages can be achieved. A transformer connected between an HVDC converter and an AC grid is typically large, and therefore costly, and generally has to be tailor-made for a particular HVDC system. Thus, by leaving out such transformers, considerable savings can be made in terms of manufacturing time, installation time and monetary expenses. Furthermore, energy losses which normally occur in such transformers will be eliminated.
Transformerless HVDC converter stations are discussed in “DC Systems with Transformer-less Converters”, IEEE Transactions on Power Delivery, Vol. 10, No. 3, July 1995, pp. 1497-1504, wherein a transformerless thyristor-based HVDC station is disclosed. This publication proposes a solution to the problem of how to block common mode direct currents from entering the AC system wherein a coupled reactor is connected in series with both DC poles. By measuring the DC flux in the core of this reactor, the presence of a DC common mode current can be detected. Based on these measurements, the control of the firing of the thyristors of the HVDC station is adapted in such way that the DC common mode current is counteracted.
However, this solution is only effective in eliminating common mode DC currents. In general, it is also required to avoid differential mode DC currents. Therefore, another way of preventing DC currents to enter the AC grid is desired.